1. Field of the Invention
The present invention relates to a system for controlling idling speed of an internal combustion engine, more particularly to a system for controlling an idling speed enabling altitude compensation at high altitudes.
2. Description of the Related Art
In the prior art, there are already known various methods for controlling the rotational speed of an engine at an idle position of a throttle valve. In one method, the idle position of the throttle valve is controlled to regulate the intake air flow rate. In another, an air control valve is arranged in an intake path for bypassing the throttle valve and the air flow rate passing through the bypassed intake path is controlled by regulating the control valve. In this case, the position of the throttle valve or the afore-mentioned air control valve is regulated, in response to the difference between the desired rotational speed for control at the time of idle running and the actual rotational speed of the engine, to control the intake air flow rate. Thus, closed loop control is effected so that the rotational speed becomes equal to the desired rotational speed for controlling.
When the engine is run at a high altitude, the weight of the intake air decreases by an amount corresponding to the reduction in the intake air density. This causes a decline in the idling speed, causing, in the worse case, engine stalling.
In the prior art, there is provided for instance, a sensor for detecting the atmospheric pressure in the neighborhood of the engine. The idling intake air flow rate is connected in response to the output of the atmospheric pressure sensor. This prior art, however, requires not only an atmospheric pressure sensor, but also a circuit for processing the output of the atmospheric pressure. This means increased production costs and an increased number of terminals in the control circuit. Reference can be made to Japanese Unexamined Patent Publication (Kokai) No. 57-131841 which describes the connection of the intake air flow rate in the idling state in response to the output of an atmospheric pressure sensor.